Olefins, (exemplified herein as, but not limited to, ethylene, propylene, butenes, and mixtures thereof), and their substituted counterparts, serve as feedstocks for the production of numerous chemicals and polymers. For example, ethylene is one of the largest volume chemical intermediates in the world, being used as a raw material in the production of, for example, polyethylene, ethylbenzene-styrene, ethylene dichloride, ethylene oxide and ethylene glycol. Most olefins are commercially produced by the thermal or catalytic cracking of saturated hydrocarbons found in petroleum and naphtha (See M. Ladisch et al., Science (1979) 205, 898). Due to the thermodynamic limitations of the reaction, thermal cracking reactors operate at temperatures as high as 1,100° C., and challengingly short reaction times, to maintain the desired levels of conversion—typical yields are between 50 and 100% (See U.S. Patent Applications and Patents: 2006/0149109; U.S. Pat. Nos. 4,351,732; 4,556,460; 4,423,270; and 4,134,926). Information on production of ethylene by thermal cracking is available in Kirk Othmer Encyclopedia of Chemical Technology, 5th ed. Wiley (2004-2007), and Ullmann's Encyclopedia of Industrial Chemistry, 6th ed. Wiley (2003), both of which are hereby incorporated by reference.
Finding new, more efficient, and environmentally friendly pathways to produce olefins from renewable starting materials that are not encumbered by the varying costs and tightening supply of crude petroleum has been a challenging research area of the past decade (See U.S. Patent Applications and Patents: 2006/0149109; U.S. Pat. Nos. 4,351,732; 4,556,460; 4,423,270; and 4,134,926). Lower alcohols, such as ethanol, propanol, and butanol, are frequently available from renewable sources and thus provide a pathway to their corresponding olefins independent of fossil fuels. Catalytic oxidative dehydration of ethane was proposed as an alternative method to produce ethylene at much lower temperatures, but the yields and selectivity achieved to date have not been encouraging (See S. Golay et al., Chem. Eng. Sci. (1999) 54, 3593).
The dehydration of oxygenates, such as alcohols, can be carried out using liquid acids, either concentrated sulfuric acid or concentrated phosphoric acid, H3PO4, as a catalyst. The mechanistic details for the dehydration reaction can be summarized in Scheme 1 (below). The alcohol is first protonated, followed by a loss of water to give a carbocation (carbonium ion), which results in the subsequent abstraction of a hydrogen ion from the carbocation. Apart from the acid's corrosive nature, as a side reaction, the acid can oxidize the alcohol into polluting carbon dioxide. Also, in the case of concentrated H2SO4, large quantities of sulfur dioxide can be produced. Both of these gases have to be removed from the product olefin before it can be used in a later chemical process.

Silicoaluminophosphates (SAPOs), such as SAPO-34 and its analogues, possess strong Brönsted acid sites and are excellent shape-selective catalysts for the conversion of methanol and other alcohols to light olefins (See U.S. Pat. Nos. 4,499,327; 5,952,538; 6,046,673; 6,334,994; and 7,199,277; as well as WO 1993/024430). However, SAPOs are composed of Si atoms tetrahedrally coordinated to oxygen atoms making an integral part of the overall catalyst framework. SAPO-34 is being commercially exploited (by UOP) for the selective conversion of methanol to low-molecular weight olefins (See WO 2007/032899). Further, the Brönsted acidity of a silicoaluminophosphate varies greatly depending on its particular structure type and architecture.
Olefins, particularly light olefins, are the most desirable products from oxygenate conversion and crude petroleum cracking. A need exists to improve the performance of ethylene and propylene plants. To this end, a number of catalytically mediated processes have been proposed. The most chemically straightforward among these is ethanol, or propanol, dehydration.
Many of the downstream industrial processes for which ethylene is the raw material, including the manufacture of polyethylene, ethylene dichloride, ethylene oxide, etc. operate at super atmospheric pressure. Processes for dehydration of ethanol to ethylene are well known. These processes typically require temperatures in excess of 300° C. where both the olefin and alcohol are in the gas phase and achieve essentially complete conversion of alcohol to olefin. The thermodynamics, however, favor such high alcohol to olefin conversions only at low pressure, so the process is conventionally operated at or just above atmospheric pressure. The ethylene produced must also meet critical purity specifications. Purification is conventionally done via cryogenic distillation at elevated pressure. Thus, if ethylene is produced by the conventional gas phase dehydration of ethanol, it must be compressed before purification. Moreover, ethylene produced from an ethanol dehydration unit must, after purification, again be compressed to the operating pressure of the eventual downstream process. Clearly, there is a need to produce ethylene by ethanol, or propylene by propanol, dehydration at elevated pressure such that its downstream use avoids such steps and becomes more economical.
In addition, renewable ethanol, a potential dehydration feedstock, is typically made by fermentation of an agricultural material in an aqueous medium. The ethanol after being separated from fermentation solids is quite dilute in water. Most of this water is conventionally removed from the fermentation broth before dehydration to form ethylene. The presence of water is thermodynamically detrimental to achieving a high conversion in gas phase dehydration. There exists, therefore, a need for a dehydration process capable of accepting aqueous ethanol as its feedstock.